Zeolites are crystalline alumina silicate materials that exhibit a highly ordered porous structure with pores of molecular diameter. IUPAC identifies this type of porosity as microporous, as the size of the pores are not wider than 2 nm. The other groups of porosity are mesoporous (pore size between 2-50 nm) and macroporous (pore size larger than 50 nm). Zeolites consist of tetrahedral TO4 units (T=Si or Al), which gives the framework an overall composition of TO2. These materials have a clear organized framework throughout the crystals, giving rise to highly ordered pores and a large internal surface area. By replacing a silicon atom with an aluminium atom, it is possible to generate a deficit of charge, which is compensated by a cation located nearby. The cation is usually an alkali metal (such as sodium), alkali earth metal, or possibly a H+ ion. If the cation is a proton, the zeolite becomes a strong Brønsted acid. All these characteristics give rise to a lot of uses for zeolites. These applications range from ion exchangers in laundry detergent powder to adsorbent agents and as catalysts, often as solid acid catalyst due to the Brønsted acid sites. Their catalytic applications range from fluid catalytic cracking (FCC), petrochemistry and the synthesis of special chemicals to environmental catalysis.
Today, nearly 60 different natural occurring zeolites are known, while 201 can be prepared synthetically [1]. These zeolites have different structures, due to different Si—O—Al linkages, and a different number of Si or Al atoms linked in each “cage”. This also creates different pore system of one-, two, or three-dimensions in the zeolite. As the pores are very regular, and around the same size in diameter as molecules, it is possible for zeolites to function as molecular sieves. Due to their chemical structure and molecular sieve properties, zeolite catalysts exhibit high selectivity for a variety of chemical reactions. Since most of the surface area and the active sites are within the zeolite, the shape of the pores and channels give rise to shape selective catalysis. Commonly there is distinguished between three types of molecular sieving effects:                1) Reactant shape selectivity: Only molecules small enough can enter the zeolite pores and undergo chemical transformation or be adsorbed.        2) Product shape selectivity: The size of the pores is too small, that not all possible products can diffuse out of the zeolite after reaction. This leads to an increased selectivity towards smaller molecules or isomers.        3) Restricted transition-state shape selectivity: Here the formation of too large transition state intermediates are prevented due to zeolite pore size. FIG. 1 illustrates the three different kinds of shape selectivity.Zeolite Synthesis        
In general, zeolite synthesis is a crystallization process, where silica and alumina species dissolve and react to give a less soluble crystalline alumina/silicate product. The crystallization process is typically performed in a hydrothermal process where these zeolite precursors are added to an autoclave and heated to relatively high temperatures and autogenous pressures. The high pressure is due to the evaporation of water inside the autoclave, and is very important for the synthesis. In a typical synthesis the zeolite precursors are dissolved or suspended in an aqueous solution of a structure directing agent (SDA) and an alkali hydroxide to catalyze the breaking and formation of chemical bonds [4].
The structure directing agents are almost always organic amine cations. Some of the most commonly used organic structure directing agents are tetramethyl-ammonium (TMA), tetraethylammonium (TEA), and tetrapropylammonium (TPA), though compounds as diverse as Choline, 1,6-diaminohexane, and hexanediol have been used. During the zeolite crystallization process, the zeolites form around molecules of the structure directing agent. The shape and properties of the structure directing agent causes the zeolites forming around it to take a certain shape. Stoichiometric analysis of samples of ZSM-5 has indicated that one TPA+ molecule occupies each intersection between pores in the zeolite [2].
For sources of silicon, mostly sodium silicate, fumed silica or tetraethoxy orthosilicate are used, while sodium aluminate, aluminum nitrate or -chloride are typical sources of aluminum [3]. The mixture of zeolite precursors (amorphous zeolite gel) is then transferred to an autoclave and heated to a predetermined temperature, often between 120-200° C. Within days, possibly weeks, the precursors begin to crystallize and form the zeolite. After the synthesis, the autoclave is cooled to room temperature, and the zeolite material is washed with water and isolated by filtration or centrifugation. The zeolite is then calcined at around 500-600° C. to remove residual SDA and framework water. At last the zeolite can be ion exchanged. This can either be done to introduce hydrons, alkali metal, alkali earth metal, lanthanoid or transition metal cations.
In 1983 Taramasso et al. incorporated titanium ions into silicalite-1 (denominated as TS-1) [56]. The incorporation of titanium is an isomorphous substitution in the MFI lattice of the silicalite-1. The presence of a titanium atom gave rise to different catalytic properties than the selective acid catalytic properties displayed by conventional alumina silicate zeolites. The TS-1 has been found useful in selective oxidation reactions, such as the hydroxylation of phenols, epoxidation of alkenes, and ammoxidation of ketones [57-61].
Diffusion in Zeolites
The microporous structure of zeolites does not only determine the chemical selectivity, but also play an important role concerning mass transport within the zeolite crystal. The micropores can limit the diffusion, molecular mobility and ultimately determine the reaction rate of the overall process [6, 7]. In addition, slow diffusion can cause polymerization of by-products or reaction intermediates blocking catalytic active sites within the microporous channels. For some catalytic applications this may lead to severe deactivation [8]. In the case of zeolites with unidirectional channels, the diffusion can be sharply reduced by small amounts of debris in the pores generated during zeolite synthesis or activation and by small amounts of strongly adsorbed molecules.
These problems are smaller in zeolites in which a bidirectional or tridirectional pore system is present [9]. By synthesising zeolites with extra-large pores, it is possible to overcome some of these mass transfer limitations. These materials are often synthesised using bulky organic amines as a pore-generating agent [10-14]. However, the unfavourable thermal stability of large pored zeolites, in combination with the high cost of these organic templates for the synthesis [14-17] hampers their use. In addition, the large unimodal mesopore system of these zeolites will not exhibit the same shape selectivity as the microporous zeolites.
Another way to improve the diffusivity is by using ordered mesoporous materials, e.g. MCM-41 and SBA-15. These silica composites contains large and uniform pores, which has proven to be effective concerning the molecular diffusion and mass transfer problems associated with conventional zeolites. These materials have shown an increased catalytic activity compared to conventional zeolites [18]. However, due to the amorphous nature of these materials, they exhibit low thermal and mechanical stability [7, 19] as well as weaker acidity compared to conventional zeolites. These drawbacks severely limit the catalytic application of both extra-large pored zeolites and ordered mesoporous materials and other possibilities must be examined.
In order to improve accessibility to the catalytically active sites in the micropores of zeolites, without losing shape selectivity and stability, much effort has been devoted to the development of hierarchical zeolites. Hierarchical zeolites are characterised by porosity in the meso- or macropore range in addition to the micropores [7]. The reason for the increased interest in these materials, is the addition of the improved transport due to the interconnections of a secondary network of inter- or intracrystalline mesopores, while still maintaining the catalytic properties (shape selectivity and hydrothermal stability) associated with the conventional zeolite. Three overall hierarchical zeolite types exist:                1) Nanosized Zeolites are zeolites that exhibits intercrystalline voids or pores, in addition to the micropores present in the zeolites.        2) Composite Zeolites are zeolites supported on a material that is typically meso- or macroporous. Here, the support is facilitating the mass transport.        3) Meso- or macroporous Zeolites are zeolites where the additional porosity has been introduced by demetallation or by using an organic template.        
Mesoporous zeolites are characterized by having connected intracrystalline meso- and micropores. The micropores originate from the conventional synthesis of zeolites, but the mesopores can be introduced in numerous ways. Overall, there are two distinctive methods to produce a mesoporous system inside a zeolite: Non-templating and templating methods. Demetallation is a non-templating method and involves dissolving part of a conventional zeolite by the use of a chemical reagent. The conditions are typically quite harsh, involving strong acids, bases, hot steam or complexing agents, as zeolites are highly stable. Regarding the templating procedures, several types of templates have been utilised for the introduction of mesopores in zeolites. These are grouped into two groups, depending on the interface between the zeolite crystal and the template.
Soft templating is the use of e.g. surfactants to generate porosity. One method is to add a surfactant to the zeolite synthesis gel. This facilitates the assembly of a mesostructured phase from the zeolite seed solution [35, 36]. After the hydrothermal synthesis the soft template is removed by combustion together with the SDA.
Hard templating applies a solid material to generate the mesopore system. This method has proved to be very effective and a highly versatile approach. Templates include organic aerogels, polymers, and carbon in different forms. Here, only carbon templates will be mentioned. One of the well-known methods is the crystallization of zeolite gel in porous carbon particles. If the amount of synthesis gel relative to the carbon template is sufficient, the zeolite crystals continue to grow after nucleation in the cavities of the carbon. This will allow the zeolite crystal to encapsulate the carbon. Combustion of the carbon particles embedded in the zeolite crystal, will lead to the formation of mesopores [37]. Several types of carbon nanoparticles have been used [38], including carbon nanotubes [39] and nanofibers.
As a carbon source it is also possible to apply in situ prepared carbon, typically by carbonization of various precursors. This has been done by e.g. decomposition of various precursors in the pores of ordered mesoporous materials such as MCM-41. The mesoporous material is then dissolved and the resulting carbon is impregnated with the zeolite synthesis gel. After crystallisation and combustion of the carbon, mesoporous zeolite is obtained [40]. Kustova et al. [41] reported a similar synthesis method, only with the use of cheap silica and sucrose as silicon and carbon precursors. This resulted in a crystalline mesoporous zeolite.
Despite the obvious advantages of mesoporous zeolites compared to conventional zeolites, it is not without disadvantages. Introducing mesoporosity in zeolites will cause an increasing portion of the active sites to be available for molecules too large to actually enter the micropore structure. It has been shown, that 2,4-dimethylpyridine probes approximately half the Brønsted acidic sites in a commercial ZSM-5 sample vs. nearly 100% after a strong desilication [54]. In addition the mesopore walls can also be catalytically active [55], allowing the inclusion and catalysis of bulky molecules. This means that shape selectivity of mesoporous zeolites will be reduced compared to microporous zeolites. The design of the mesoporous zeolite must therefore be carefully considered, before it is applied to a given reaction.
Despite the growing demand, a fast, efficient and economically feasible process for synthesising mesoporous zeolites or zeotypes (ie. artificial materials based on the structure of zeolites) that can be scaled up for industrial application has not yet been reported.